This invention relates to the separation of solids and liquids and, more particularly but not exclusively, is concerned with a method of pressure filtering a mixture of a particulate solid and a liquid to obtain a filter cake of the particulate solid which has a low liquid content.
The liquid content of a filter cake formed when a mixture of a particulate solid and a liquid, for example in the form of a slurry, is subjected to pressure filtration decreases as the pressure applied to the slurry during filtration is increased. However, even when the applied pressure is very high, i.e. in excess of 1000 pounds per square inch gauge (p.s.i.g.) or more, the filter cake inevitably retains a certain amount of liquid in the interparticle pores within the filter cake. The amount of liquid retained depends primarily upon the porosity of the filter cake which in turn is a function of the applied pressure, the particle size distribution of the material which forms the filter cake, and the density of the particle packing.
In recent years there have been developed what are frequently known as tube pressure filters which are capable of operating at pressures in excess of 1000 pounds per square inch (p.s.i.). One kind of tube pressure filter comprises an arrangement of (a) a pair of generally coaxial tubular assemblies arranged one within the other, (b) an impermeable elastic sleeve disposed within and secured to the outer of said tubular assemblies, (c) a filter element disposed around and supported by the inner of said tubular assemblies, (d) first discharge means for the discharge of filtrate which has passed through the filter element, and (e) second discharge means for the discharge of solid material retained on said filter element, the arrangement being such that in a first operating condition of the tube pressure filter said second discharge means is closed and the tubular assemblies cooperate with each other to define a closed annular chamber which is divided into generally coaxial and non-intercommunicating inner and outer compartments by said impermeable elastic sleeve, the inner compartment having an inlet for a feed slurry comprising a mixture of liquid and a particulate solid and the outer compartment having an inlet for a hydraulic fluid under pressure, and in a second operating condition of the tube pressure filter said second discharge means is open to enable particulate solid in the form of a filter cake to be discharged from the inner compartment. Herein this kind of tube pressure filter will be referred to as "a tube pressure filter of the kind set forth". Embodiments of tube pressure filters of the kind set forth, and details of the construction and operation thereof, are given in, for example, British Pat. Specifications Nos. 907,485; 1,271,494; 1,240,465; 1,240,466; 1,317,887, 1,340,858; 1,351,142; 1,351,943; 1,355,334; 1,336,856; 1,362,655; and 1,386,256; and U.S. Pat. No. 3,900,403.
It has been suggested that dewatering of the filter cake might be assisted by passing air through the filter cake after it has been formed on the filter element and when substantially no more liquid can be expressed from the filter cake by application of the pressure exerted by the hydraulic fluid. However, there has not been described a satisfactory way of achieving this object.